Cryptography in Network Communications

28 An example of a hash would be to take an input password, multiply it by 3.1415, divide by e 2.71828, mod the result by 7654321, and take the middle eight bytes. It would certainly be nasty to reverse this process without knowing anything about it. The NISTs National Institute of Standards and Technology proposed message digest function hash is called SHA, which stands for Secure Hash Algorithm. Ron Rivest hes the R in RSA created a set of hash algorithms, MD2 through MD5, which stand for Message Digests.

2.2.4.2 Secret key systems

The secret key cryptosystem takes as input a message of a variable length and a secret key by which the message is transformed into the ciphertext. In fact, from a distance the transformation is very similar to the hash algorithms. An important distinction is that a separate user-controlled variable is supplied to help encipher the data. Further, where the hash algorithm creates a fixed length result from a variable length input, the secret key system operates on successive fixed blocks of input using the fixed length key to produce a variable length result. Hash systems are for one-way checks, and secret key systems preserve the entire clear text so that the original text can be produced when needed. Because the secret key used in the equation is of a fixed length, using a key that is too short reduces the overall security of the system. Imagine using a key that is one byte long 8 bits; it shouldnt take a cryptoanalyst too long to run through the 256 possible keys that could be used to decrypt the data. Since the operation of encrypting the message uses fixed blocks of input, using a message block size of one byte would also be insecure because the cracker would only have to create a table of clear text to ciphertext pairs, once he knows that he could send his own data through the algorithm. As with hashing systems, the secret key system is designed to take a variable length clear text input and produce a random-looking similarly sized output. Further, changing the input by only one bit should change the output so that there is no way to trace a transformation from bit to bit. The randomness of the ciphered message suggests that at any given time, about half of all the bits in the resulting sequence are on and the other half are off. Secret key cryptography uses algorithms that effectively disperse the bits completely across the resulting output, and then mix them up by looping multiple times so that it becomes impossible to trace a given bit through the process and have any idea of what happened to it along the way. Typically, during the encryption process, there are several operations that can be found in use, including the substitution of input bits for other input bits, and the swapping of bit positions with other bit positions. DES, which stands for the Data Encryption Standard, was developed in 1977 by the National Bureau of Standards for low-grade U.S. government work and commercial applications. The standard was based on work done earlier by IBM that was codenamed the Lucifer Cipher. DES uses a 64-bit key, but trims the last bit off each of the eight bytes 8 bits each as a parity check, making the actual key size only 56 bits. Originally, DES was designed to be used in a hardware-only implementation, but since there has been phenomenal growth in semiconductor speeds in just the last few years, it is now just as practical to conduct in software. This was obviously beyond the intentions of its designers, who had their own agendas in mind.